It is known to provide over-center clutches equipped with adjustment devices which compensate for clutch plate wear. Some of these adjusters are manually reset when a decrease in clutch engaging effort or a decrease in clutch transmitted torque is noticed by the operator. In these particular prior art devices, resetting is generally accomplished with the clutch in the disengaged and stopped position and access to the adjusting mechanism must be provided for. The adjustment procedure usually consists of disengaging a locking device and turning an adjusting nut which reduces the distance between the clutch actuating mechanism and the clutch back plate, thereby compensating for clutch plate wear. It is furthermore usually necessary to measure clutch engaging effort after the adjustment has been made in order to assure proper clutch torque.
The prior art automatic clutch and brake adjusters sense clutch or brake clearance or stroke to compensate when these distances become excessive. An example of a prior art automatic adjuster is shown in the U.S. Pat. No. 3,752,286 which issued Aug. 14, 1973 and is entitled "Self-Adjusting Clutch". The device of that patent relates to a spring loaded clutch and the automatic adjustment is desirable in order to avoid excessive actuator stroke. In that patent device, the clutch torque or clamp-up force is maintained by springs and consequently the required accuracy of sensing and adjusting is quite minimal and an accuracy of about 0.030 inches is immaterial and direct sensing of such distances is not difficult.
Stated otherwise, some of these prior automatic adjusting devices are in part, relative position sensing units. In an over-center type clutch, the amount of adjustment is usually very minimal, perhaps on the order of 0.002 to 0.005 inches, and a position sensing device that could reliably detect and compensate for such changes would need to be very accurate and would not be entirely reliable due to machining variations, heat distortion of the clutch plates, and wear.